1. Field of the Invention
The present invention relates to semiconductor wafer manufacturing processes, and in particular, a loadlock chamber for efficient transfer of a wafer cassette from its transfer pod to internal processing stations in a contaminant free environment.
2. Description of the Related Art A standardized mechanical interface (SMIF) system proposed by the Hewlett-Packard Company is disclosed in U.S. Pat. Nos. 4,532,970 and 4,534,389. The purpose of a SMIF system is to reduce particle fluxes onto semiconductor wafers during storage and transport of the wafers through the semiconductor fabrication process. This purpose is accomplished, in part, by mechanically ensuring that during storage and transportation the gaseous media (such as air or nitrogen) surrounding the wafers is essentially stationary relative to the wafers and by ensuring that particles from the ambient environment do not enter the immediate wafer environment.
The SMIF system provides a clean environment for articles by using a small volume of particle-free gas which is controlled with respect to motion, gas flow direction and external contaminants. Further details of one proposed system are described in the paper entitled "SMIF: A TECHNOLOGY FOR WAFER CASSETTE TRANSFER IN VLSI MANUFACTURING," by Mihir Parikh and Ulrich Kaempf, Solid State Technology, July 1984, pp. 111-115.
Systems of the above type are concerned with particle sizes which range from below 0.02 .mu.m to above 200 .mu.m. Particles with these sizes can be very damaging in semiconductor processing because of the small geometries employed in fabricating semiconductor devices. Typical advanced semiconductor processes today employ geometries which are one-half micron and under. Unwanted contamination particles which have geometries measuring greater than 0.1 .mu.m substantially interfere with 1 .mu.m geometry semiconductor devices. The trend, of course, is to have smaller and smaller semiconductor processing geometries which today in research and development labs approach 0.2 .mu.m and below. In the future, geometries will become smaller and smaller and hence smaller and smaller contamination particles become of interest.
A SMIF system has three main components: (1) sealed pods, having a minimal volume, used for storing and transporting cassettes which hold the semiconductor wafers; (2) enclosures placed over cassette ports and wafer processing areas of processing equipment so that the environments inside the pods and enclosures (after having clean air sources) become miniature clean spaces; and (3) a transfer mechanism to load/unload wafer cassettes from a sealed pod without contamination of the wafers in the wafer cassette from external environments.
The wafer-carrying cassettes are stored and transported between processing stations in the pods. Many of the wafer fabrication processes are carried out in a high vacuum environment. Before entering the ultra high-vacuum processing station, the wafer-carrying cassettes are transferred from the pods to an intermediate vacuum buffer area. This buffer area, known as a loadlock chamber, serves as a buffer for wafer storage and is a gateway for wafer distribution to the vacuum related processes. The effectiveness of a loadlock chamber is critical to wafer fabrication and can have a significant impact on the cost of wafer manufacturing and the quality of the finished product.
There are several ways to load a cassette into a loadlock chamber, the two most common are manual and mechanical transfer. Manual loading can easily be done if the cassette is of manageable size, but if the cassette is large or heavy, manual loading has certain limitations. One limitation is ergonomics. Continued loading and unloading motion by operators is an industry wide concern because of repetitive motion injury. Another limitation is the risk of wafer damage due to human error. A cassette of wafers can be dropped or bumped resulting in the loss of hundreds of thousands of dollars. If manual loading is to be used, provisions must be made for "elbow room" and hand clearance both in the loadlock doorway and within the loadlock chamber. Consequently, conventional loadlock chambers that are manually loaded must be made larger than necessary for just the wafer cassettes alone. Minimizing the size of a loadlock chamber is important both because space within a clean room environment is critical, and larger loadlock chambers take longer to pump-down to a high-vacuum environment. Additionally, large loadlock chambers are expensive to fabricate. FIG. 1 shows a conventional manual loading system where an operator is transferring a plurality of wafers 6, through an access port 7, and into a conventional loadlock chamber 8. Clearance within the access port and chamber beyond that necessary for the cassette alone must be provided so that the operator may grasp and properly place the cassette within the chamber. Some floor space for the cassette extracting unit is also required for removing the cassette from the SMIF-pod.
Another method of cassette loading involves the use of a mechanical transfer device for placing the cassette inside the chamber 8. These devices function similarly to their human counterpart in that they require floor space and gripping clearance. As such, they suffer the same disadvantage as manually loaded systems in that they require larger loadlock chambers than necessary to accommodate a wafer cassette alone. FIG. 2 illustrates a conventional swing-arm type device 9 between the cassette extraction unit 11 and the loadlock chamber 8.
Both manual and automated loading of cassettes into a loadlock chamber additionally require an extraction device 11 for removing the cassette from the SMIF-pod prior to the chamber loading.
With current wafer cassette loading methods, the need for strict environment control is essential for minimizing wafer contamination. As shown in FIGS. 1-2, conventional loading methods load a cassette into the loadlock chamber through an access port 7. Currently, no system exists for loading a wafer cassette into a loadlock chamber directly from an environmentally sealed pod without exposing the wafers to the surrounding atmosphere. Between the extraction sequence where the cassettes are removed from the pods, and the loading sequence where the cassettes are loaded into the loadlock chamber, the cassette is exposed to the outside air and special precautions are generally required to minimize contamination during this stage. For example, if the environment is of a lower clean room classification than the requirements for wafer processing, a special air management device is needed to ensure wafer cleanliness. These air management devices range from air curtains to full enclosures that surround the loadlock equipment. FIG. 3 shows a conventional filtration system that will provide a degree of clean laminar air flow for the cassette and wafer extraction area. This type of set-up is expensive to implement and maintain, and it does not completely isolate the wafers from the surrounding particulate contaminants. Moreover, such filtration systems are unable to prevent airborne molecular contaminants, such as water vapor and vapor-phase contaminants, from entering into the loadlock chamber, where they can contaminate the semiconductor wafers. Systems for minimizing contaminants are becoming increasingly more important as wafer manufacturing processes presently require control of particle sizes as small as 0.05 to 0.5 microns.
The above described methods for loading cassettes were developed in the early days of semiconductor processing where manual handling was the prime mode for transferring cassettes from one point to another. In the pioneering days the wafer diameters were as small as two inches across. As the semiconductor industry grew, the wafer size increased. Unfortunately the increase in wafer size also made cassettes heavier and more difficult to handle. The current 200 mm wafer size is over 15 times heavier than the 2 inch wafers. With 300 mm diameter wafer processing coming in the near future, wafer handling by conventional methods will be even more difficult due to the added weight and bulk. Moreover, present systems for minimizing contaminants within the wafer and cassette extraction area are costly and not wholly effective in preventing contaminants from reaching the wafers during transport from the pods to the processing stations.